In the GSM system, each base station is assigned a group of transmitting frequencies for communication channels. The base station can switch at regular intervals between such transmitting frequencies in a predetermined sequence. Each mobile telephone in the service zone of a base station receives data from the base station to enable it to establish a pattern of frequency-hopping to receive data transmitted in its communication channel.
The mobile telephone receiver has several control loops which track, for example, timing, frequency and signal strength of the bursts and operate to ensure that the receiver remains in synchronism with the transmitter and to provide a usable signal to the following stages.
In the particular case of an automatic gain control loop, for example, changes in signal strength are a result of normal attenuation dependent on the distance between transmitter and receiver, shadow fading effects caused by a physical object blocking the direct path and Rayleigh fading caused by the receiver moving through multiple reflected signals. Shadow fading has a spacial correlation which may be assumed to be unity over many wavelengths. The Rayleigh fading, however, may be totally uncorrelated between bursts.
It is usual to exclude the effects of Rayleigh fading by averaging over a plurality of bursts.
A problem arises, however, if co-channel interference is present at one of the hopping frequencies. Such interference will make the signal strength at that frequency greater although the signals being received are, in fact, unwanted. Gain will be reduced for all frequencies.